I. Field of the Invention
The present invention is directed to drag bits having diamond or other hard cutter inserts. More specifically, this invention is directed to tungsten carbide matrix type blade drag bits incorporating polycrystalline diamond compact (PDC) cutters when drilling very elastic or plastic abrasive earthen formations. Under such conditions of drilling, the elastic rebound and/or plastic deformation of the rock being drilled causes the abrasive rock to bear on and wear away the bit face blade material circumferentially rearward of the PDC cutters mounted in sockets on the blades. When significant wear occurs, the PDC cutters have insufficient back support to maintain the cutters in place. It is an object of this invention to prevent or minimize the wear of the bit body material behind the PDC cutters to maintain the back support of the cutters thereby preventing catastrophic cutter loss and the subsequent termination of bit life.
II. Description of the Prior Art
There are a number of diamond drag bit patents that appear to be somewhat similar in construction to the present invention, but are designed to serve a different function.
For example, U.S. Pat. No. 4,718,505 describes a steel body drag bit having stud type polycrystalline diamond cutters (PDC) affixed to essentially radial raised rib portions of the bit cutting face with drilling fluid channels formed between the ribs. Spaced essentially rearward from each cutting element is a separate abrasion element comprising a cylindrical tungsten carbide stud impregnated at the lower end with diamond particles. This abrasion element is mounted in a socket on a raised portion of the bit cutting face and protrudes a significant amount from the rib outer surface, but a lesser amount than does the PDC cutting element. This abrading element may be located rearwardly behind the PDC cutter element on the same raised rib portion or on a separate rib with a fluid channel therebetween. This separate abrading element is intended to act as a back-up cutter in the event of the leading PDC cutter wear or breakage.
While bits built by the teaching of this patent have proven to be satisfactory in increasing bit life, the drilling rates are considerably slower when the abrading elements take over the drilling function because they are much less aggressive than sharp PDC cutters.
Another patent, U.S. Pat. No. 4,889,017, assigned to the same assignee as the foregoing patent, describes a polycrystalline diamond drag bit with the cutting head fabricated from a powdered tungsten carbide matrix material. The overall geometry of this bit type is essentially the same as described in the previous patent except for the method in which the abrading elements are formed. The tungsten carbide matrix head is cast in a refractory mold, such as graphite, by methods well known to those skilled in the art. The protruding abrading elements impregnated with diamond particles are formed as an integral part of finished cast cutting head. Bits made according to the teachings of this patent also offer additional bit life after the primary PDC cutters are worn out or broken, but the drilling rates are significantly slower than bits with intact primary cutters. Such bits also sustain breakage of the rearward edge of the raised rib with subsequent loss of primary PDC cutters because of the rearward tensile and shear forces imposed by the overturning movement of the protruding abrading element.
Still another patent, U.S. Pat. No. 4,991,670 is a Continuation-In-Part of U.S. Pat. No. 4,889,017, therefore the same advantages and disadvantages are applicable.
The present invention overcomes the shortcomings of the foregoing prior art patents by providing a single layer of diamond or other ultra-hard and abrasion resistant pieces imbedded in the tungsten carbide matrix ribs rearward of the PDC cutters. The ultra-hard pieces have at least one flat surface which is positioned with the flat surfaces flush with the outer surface of the bit body raised ribs. This provides an excellent wear or abrasion resistant surface having a very low coefficient of friction. Being flush set, the ultra-hard pieces do not engage the rock formations as cutting elements, therefore very low tensile and shear stresses are imposed on the rearward edge of the brittle raised ribs, thereby eliminating or minimizing wear and breakage of the trailing rib surface.